suggestions

[747mopar]

Sorry for yet another carb question but I'm against the wall with time tonight with the Charger being the only car I have to haul my kids around in for the next couple of days. I've been tinkering with the new Demon 850 and have the running really well accept for a low rpm miss. It runs perfect at idle and 2,000 and above but now with the new tranny pulling the rpms down into the 1,500 rpm range it has a continuous miss. I've already tried the timing to no avail and have been reading my books AGAIN. So my question is, what's the chances of the carb still being in the idle screw circuit at that rpm and with the power valve being for WOT and heavy loads would 1,500 rpm be a heavy enough load that I may need to change it? Currently the power valve is a 6.5.

 

[bigmanjbmopar]

Have you tried a 4.5 valve?

 

[747mopar]

All I've tried so far is the timing, raising the float level and cleaning the carb up really good. I've only gotten to drive it twice this year and all of a sudden I have depend on it now so I'm just trying to nail it down quickly. I have a 4.5 but that power valve really made my last carb (870 holley) run terrible?

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Wouldn't a 4.5 make it worse if it's lean which it seams to be? A 4.5 should make the power valve come in at an even lower (later) vacuum instead of earlier wouldn't it?

 

[bigmanjbmopar]

Probably not, hard to say those small off the line stumbles could be floats but if you have adjusted them then who knows. Try jetting down see if that helps. Personally I would go with a 4.5 and down 2 jet sizes for a streetable ride. an 850 is pretty big, I know my QFT 850 tends to stumble ever so slightly off idle with a 6.5. Now run a 780 vac sec. much improved daily drivability.

 

[super-bee_ski]

Your quote: Wouldn't a 4.5 make it worse if it's lean which it seams to be? A 4.5 should make the power valve come in at an even lower (later) vacuum instead of earlier wouldn't it?

To mean, "Lean' or 'not firing" properly means you are not getting enough fuel.

 

[benno440]

Why not ge the car a full dyno tune, it will love you for it

 

[747mopar]

Why not ge the car a full dyno tune, it will love you for it

Trust me that's always been the plan but being a man with a family I have to watch my spending. That was going to be the first thing on the list this Spring but my tranny upgrade ate up all my funds so it's more backyard tuning for now.

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Your quote: Wouldn't a 4.5 make it worse if it's lean which it seams to be? A 4.5 should make the power valve come in at an even lower (later) vacuum instead of earlier wouldn't it?

To mean, "Lean' or 'not firing" properly means you are not getting enough fuel.

Exactly, that's why I question opening the valve even later making it stay in a lean state longer than it already is. If I understand it correctly a 6.5 means it will open once vacuum drops below 6.5 whereas it will have to drop all the way to 4.5 with the 4.5? Correct me if I'm wrong because I'm sure you guys have been at it longer than me.

 

[coloradodave]

To try and pin it down a little closer: You said it has a low RPM "miss". A miss is usually one cylinder dropping out. A "stumble" is a different problem. I would start by hooking up a dwell/tach that is easy to read fluctuations in RPM under the hood. Pull one spark plug wire at a time and see how much each cylinder causes the RPM to drop. Hopefully, there will be one cylinder that doesn't drop it as much as the rest, or not at all. This is called a cylinder balance test and can isolate a "miss" problem to one cylinder, wire, plug or cap terminal. If you find one, try switching the plug and wire and see if the problem cylinder 'moves' with the parts. Good luck to you...

 

[bigmanjbmopar]

Give it a shot if you have the jets and valve, you can richen it up at idle better.

 

[747mopar]

To try and pin it down a little closer: You said it has a low RPM "miss". A miss is usually one cylinder dropping out. A "stumble" is a different problem. I would start by hooking up a dwell/tach that is easy to read fluctuations in RPM under the hood. Pull one spark plug wire at a time and see how much each cylinder causes the RPM to drop. Hopefully, there will be one cylinder that doesn't drop it as much as the rest, or not at all. This is called a cylinder balance test and can isolate a "miss" problem to one cylinder, wire, plug or cap terminal. If you find one, try switching the plug and wire and see if the problem cylinder 'moves' with the parts. Good luck to you...

Thanks Dave, I'm not the best at terminology so I'll try a little better. The motor idles smooth as butter and runs perfectly until you drop it in 6th where it's held at 1,500 rpm or any combination of speed and gear that equals 1,500 rpm, then it stumbles (stumble is probably a better description than miss) but a little throttle brings it right out of it. Driving around town it's almost unnoticeable since your usually accelerating and the accelerator pump will bridge the lean spot as long as you accelerate past the rpm range, it's only when you hold the rpm or accelerate slowly at which time there's a small gasp in the acceleration. Hope that is a better explanation.

I can try the test you describe but I'm not noticing any hiccups, stumbles or misses at idle just right off idle (just barely touching the pedal and it only seams to do it under load).

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Give it a shot if you have the jets and valve, you can richen it up at idle better.

Kids didn't give me a chance to tackle it today ................. They had to go get ice cream LOL. I'll keep that in mind tomorrow and I found my Demon book so I'm brushing up a bit.

 

[bigmanjbmopar]

Well before you go tearing a perfectly good carb apart. listen for a miss in the spark, I had a spark plug that had a crack in the insulator and it was causing this stupid minor miss at about 1200 - 1800 rpm could watch it on the tach used to drive me nuts until I went to change plugs from champs to NGK thats when I found it, also found a small pinhole in a plug boot when the spark would jump to my headers,

Just saying an easy way to check is at night in the dark un the motor and look for a fly spark. Good luck!

 

[Meep-Meep]

To try and pin it down a little closer: You said it has a low RPM "miss". A miss is usually one cylinder dropping out. A "stumble" is a different problem. I would start by hooking up a dwell/tach that is easy to read fluctuations in RPM under the hood. Pull one spark plug wire at a time and see how much each cylinder causes the RPM to drop. Hopefully, there will be one cylinder that doesn't drop it as much as the rest, or not at all. This is called a cylinder balance test and can isolate a "miss" problem to one cylinder, wire, plug or cap terminal. If you find one, try switching the plug and wire and see if the problem cylinder 'moves' with the parts. Good luck to you...

Do exactly this ^^^^ But pull wires out of the cap end so you don't end up holding a live wire. Likely you have an ignition issue. Another thing to try is check the timing at tip in with vacuum advance connected and disconnected.

 

[747mopar]

Easy enough, I have a friend that will likely have the tach and I'll grab my electrical gloves from work so I don't get nailed. While I'm looking into that what's an easy way to check the spark output? I have a Mallory 6AL that I've never been to impressed with (spark is blue but looks a little weak), it did run fine all last summer though. I thought maybe the spark would be weaker since they (6AL) spark multiple times meaning less time to soak the coil but what do I know. Another thing that has always struck me as odd is how much initial it likes (36). Dialing it back makes it sluggish off the line but at 36 it pulls hard. I've got Taylor 8.2 Thundervolt wires on it........ (accept one), accidently ran one threw the belt and had to put an MSD wire on one of them but have a replacement now so I'll get it on too. Thanks for the pointers, I'll likely look into it tonight.

 

[747mopar]

Thanks guys, your suggestions made allot of sense so after all of your suggestions checked out I pulled the cap and noticed the reluctor gap was nearly non existent. Not sure how it moved since I set it last summer but resetting it seams to have cleaned it up, didn't get a chance to get it out on the freeway so I still need to test that but I think it's fixed.


NOTE....... Wearing leather gloves (forgot to grab my electrician gloves) is not acceptable by any means for pulling plug wires! Wow I can tell you from experience now that a 6AL really, really hurts haha.

 

[Roger63]

I say have a good bourbon or3 and think adout it some more

 

[747mopar]

I say have a good bourbon or3 and think adout it some more

Hmmmmmmmmmm, If what I said is completely off then by all means educate me instead of posting a completely useless reply! I'm assuming from your post the air gap being off wouldn't cause the issue I was having? If not the only other thing I did was replace a temporary plug wire I had on it that was a good MSD wire and played with the idle screws but found where I had them was spot on according to the vacuum gauge. The bottom line is something I did fixed it and if it wasn't the gap then I'd like to know.

 

[Hemi Rebel]

I would put a vacuum guage on it and drive around. See what its reads when it stumbles and misfires. Sorry, im not familiar with your setup. But chugging around at 1500 rpm with an 850 demon on top of? With what cam? A RV cam i hope.

 

[747mopar]

It's a Comp 268 and the problem is fixed, it does alright at 1,500 rpm but a set of 4:10's are on the list which will put it right where it needs to be. Thanks